Geoffrey (jĕfˈrē) [key], 1158–86, duke of Brittany (1171–86); fourth son of Henry II of England. Betrothed (1166) to Constance, heiress of Brittany, he was recognized as heir to the duchy in 1169 and succeeded to it on the death of her father. He married Constance in 1181. With his brothers he rebelled (1173–74) against Henry II and was subsequently involved (1182–84) in territorial disputes with his brother Richard (later Richard I). Geoffrey died in Paris while forming an alliance with Philip II of France against Henry II.